This invention relates to an apparatus for lapping the external circumferential face of workpieces, particularly the running face of piston rings which, for this purpose, are combined into a stack. The apparatus comprises an axially slit hollow body whose inner face constitutes the lapping surface of the apparatus.
During the lapping operation, the workpiece slides, with its surface to be polished, on the corresponding counterface (lapping face) of the lapping tool and, during this occurrence, the loosely applied lapping agent between the two relatively moving surfaces performs its lapping (polishing) function. The abrasive granules contained in the lapping agent slide and roll between the two relatively moving surfaces and thus exert both friction and pressure thereon. During this operation, one part of granules becomes embedded into the tool and into the workpiece and are held fixedly therein so that they will have a scraping and cutting effect on the respective other surface. The auxiliary material contained in the lapping agent such as fat, oil or other liquids enhances the lapping effect. The removal of material from the workpiece during this process is thus intentional and results in the desired lapping effect; in contradistinction, the material removed during this process from the tool represents undesirable tool wear.
For an arcuate lapping (that is, for a lapping of external faces of arcuate shape) closed sleeves have been used as the lapping tools. Such tools are, for reasons discussed above, exposed to a relatively large wear. Further, lapping dies or lapping rings, have also been used for arcuate lapping. These tools are reciprocated linearly on the rotating workpiece. Such a lapping process is described, for example, in W. LATZIG, LAPPEN-GRUNDLAGEN AND PRAKTISCHE ANWENDUNG (Lapping--Principles and Practice), published by Carl Hanser Verlag, Munich. (1950)
Lapping dies essentially comprise a die block having a bore which corresponds to the workpiece diameter and at least one slot extending radially with respect to the bore axis. With the aid of a setscrew threadedly engaging the die block and bridging the slot, the lapping die can be adjusted during the lapping operation as required. A lapping ring, on the other hand, comprises a rigid slotted ring whose axial length is about 1/4 shorter than the length of the workpiece, a holder which surrounds the slotted ring as well as adjusting screws which are arranged at the outer circumference of the holder. In case the dies or rings are arranged in an axial series, as it is necessary for certain operations, significant difficulties are encountered regarding the adjustment of these lapping tools, because each individual lapping die or lapping ring has to be separately set. Thus, the extent of adjustment would depend in a large measure, if not entirely, from the personal judgment of the operator who adjusts the lapping dies or lapping rings. Since the workpieces are packaged in stacks with an axial play for obtaining a slightly spherical contour of the lapped surfaces, it would depend on pure coincidence to locate a jammed workpiece (because of the excessive adjustment of the lapping die or lapping ring) for loosening the corresponding lapping die or lapping ring.